A. Golf Ball Construction and Composition
Modern golf balls generally comprise a core and at least one additional outer layer. Ball performance can be modified by changing the ball construction and/or composition of one or more golf ball components. Two-piece balls, having a cover around a solid, spherical rubber core, provide high initial speeds but relatively low spin rates. A three-piece ball has one additional layer between the core and outer cover layer, a four-piece ball has two additional layers between the core and outer cover layer, and so on. Chemical composition can be modified to vary, for example, polymer hardness, compression, resilience and/or durability. Most modern golf balls are multi-layered balls having cores comprising synthetic rubbers based on polybutadiene, especially cis-1,4-polybutadiene.
B. Golf Ball Compositions Comprising Ionomers
A polymer with ionic groups is generally referred to as a polyelectrolyte. Ionomers, a particular type of polyelectrolyte, typically are copolymers that contain both nonionic repeat units and a small percentage, generally less than about 15%, of ion-containing repeat units. One example of an ionomer is the sodium or zinc salt of poly(ethylene-co-methacrylic acid), a copolymer derived from ethylene and methacrylic acid.
Non-polar chains of an ionomer group together, as do the polar ionic groups, which produces ionomers having physical properties similar to crosslinked polymers or block copolymers, although ionomers are not crosslinked polymers in the traditional sense. When heated sufficiently to overcome the electrostatic forces causing the ionic groups to cluster, ionomer polymer chains move more freely. This results in a polymer having elastomeric properties and the processability of a thermoplastic, and hence such polymers often are referred to as thermoplastic elastomers. Whether an amorphous polymer is a thermoplastic or an elastomer depends on its glass transition temperature, Tg, which is the temperature above which a polymer becomes soft and pliable, and below which it becomes hard and glassy. If an amorphous polymer has a Tg below room temperature, that polymer will be an elastomer, because it is soft and rubbery at room temperature. If an amorphous polymer has a Tg above room temperature, it will be a thermoplastic, because it is hard and glassy at room temperature.
Some known polyurethane ionomers have cationic sites, and are referred to as “cationomers,” whereas others have anionic sites, such as carboxylate and sulfonate groups, and are referred to as “anionomers”. Protonated polyurethanes (polyurethane ionomers) are referred to as “acid-form polyurethanes.” Ionomers can be formed from acid-formed polyurethanes by reaction with a basic metal ion source. Polyurethane ionomers are described in many publications, including “Advances in Urethane Ionomers” (Edited by H. X. Xiao and K. C. Frisch, Technomic Publishing Company, 1995, ISBN No. 1-56676-289-8); S. A. Chen and J. S. Hsu, Polymer 34 (1993), 2769; E. Zagar and M. Zigon, Polymer, 40 (1999), 2727; and C.-Z. Yang, T. G. Grasel, J. L. Bell, R. A. Register and S. L. Cooper, J. Polym. Sci., Part B: Polym. Phys., 29 (1991), 581. Some known polyurethane cationomers are described in the following representative references: W. C. Chan and S. A. Chen, Polymer, 29 (1988), 1995; S. Mohanty and N. Krishnamurti, J. Appl. Polym. Sci., 62 (1996), 1993; J. C. Lee and B. K. Kim, J. Poly. Sci., Part A: Polym. Chem., 32 (1994), 1983; X. Wei, Q. He and X. Yu, J. Appl. Polym. Sci., 67 (1998), 2179; and Shenshen Wu and Murali Rajagopalan, WO 96/40378, U.S. Pat. No. 5,692,974, and references therein.
C. Patented Golf Ball Compositions Comprising Ionomers
A number of issued United States patents disclose particular ionomers ostensibly useful for making golf balls. Ionomers initially were formed by fully or partially neutralizing carboxyl functional groups with metal cations. This method is exemplified by Yuki et al., U.S. Pat. No. 4,955,966, “Rubber Composition and Golf Ball Comprising It.” A brief historical review of the subsequent development of ionomers for use in golf balls is provided by Radjagopalan, U.S. Pat. No. 5,691,066, “Golf Ball Comprising Fluoropolymer and Method of Making Same,” and Dalton et al., U.S. Pat. No. 6,653,403, “Golf Balls Having a Cover Layer Formed from an Ionomer and Metallocene-Catalyzed Polyolefin Blend and Methods of Making Same.”
Shenshen Wu is the named inventor on several U.S. patents assigned to Acushnet Company, of Fairhaven, Mass., that concern ionomer compositions ostensibly useful for making golf balls. See, for example, U.S. Pat. No. 5,484,870, entitled “Polyurea Composition Suitable for a Golf Ball Cover,” which discloses reacting particular organic isocyanates with particular diamines, such as the following compound.
Shenshen Wu defines X to be “2-12 carbon alkylene or cycloalkylene groups. The alkylene and cycloalkylene groups represented by X may be substituted or unsubstituted.” The '870 patent, column 2, lines 62-64. Polymer compositions that are made using particular diamines, or aminobenzoates, are disclosed in U.S. Pat. Noss. 6,117,024, entitled “Golf Ball with Polyurethane Cover”; U.S. Pat. No. 6,610,812, entitled “Golf Ball Compositions Comprising a Novel Acid Functional Polyurethane, Polyurea, or Copolymer Thereof”; U.S. Pat. No. 6,951,519, entitled “Thermosetting Polyurethane Material for a Golf Ball Cover”; and U.S. Pat. No. 6,903,178, entitled “Acid-functional Polyurethane and Polyurea Compositions for Golf Balls.”
Another group of issued United States patents disclose particular species of compounds considered useful for making golf balls. These patents are exemplified by U.S. Pat. No. 5,691,066, entitled “Golf Ball Comprising Fluoropolymer and Method of Making Same,” which discloses making fluoropolymers having varying fluorine substitution amounts. U.S. Pat. No. 6,762,273, entitled “Thermosetting Polyurethane Material for a Golf Ball Cover,” is but one of several United States patents exemplifying ionomers that are derived from diaminodiphenylmethane.